The present invention relates to a method of controlling holding pressure retention in an injection molding process and an apparatus therefor.
Injection molding is performed under various temperatures due to factors such as a temperature change of seasons, a temperature change between day and night, and so on. These temperature changes mainly affect the temperature of a metal mold or of melted resin which is a molding material to be injected into the metal mold, so that the heat history of the resin is changed in the process where the resin is charged into the mold, cooled and solidified therein, and then taken out from the mold. The change of the heat history of the resin in the mold changes the viscosity and density of the resin, and, as a result, for example, produces a change of resin pressure in the mold. Therefore, a change is produced in the weight and size of molded products, so that the quality of molded products is not consistent.
A known method of solving such a problem caused by temperature changes utilizes a pressure sensor provided in a mold, resin temperature in the mold is detected by the sensor or calculated from the thickness of a molded product, effective heat diffusion rate, and so on, and the specific volume of a molded product is controlled to be a desired value regardless of the temperature change on the basis of PVT property data which is basic physical-property data expressing the relationship among the pressure (P), the specific volume (V) and the temperature (T) of the molding material (resin) (see Japanese Unexamined Patent Publication Nos. Sho. 63-3926, and Sho. 63-3927).
In each of the above-mentioned conventional methods, it is necessary to attach a pressure sensor in a mold, which is not economical. Also, it is difficult to adjust and attach the sensor. In addition, since a conventional calculation expression of in-mold resin temperature is approximate, the accuracy of calculation is not high so that the quality of molded products cannot be controlled with a high accuracy. In addition, additional data such as the thickness of a molded product, the effective heat diffusion rate, and so on are required, and it is troublesome to collect this data.
In addition, in injection molding, for the purpose of process controlling, process monitoring, and so on, it is necessary to control the injection speed, the pressure retention force, and so on, correspondingly to the in-mold resin temperature. As a method for obtaining the in-mold resin temperature, conventionally known are the following methods:
(1) A method in which a temperature sensor is provided in a cavity portion so as to directly measure the in-mold resin temperature; and
(2) A method in which an unsteady state heat condition analysis technique is used to thereby estimate the in-mold resin temperature from the following calculations a, b or c:
a. Calculation by numerical analysis by calculus of finite differences; PA1 b. Calculation by a calculation expression obtained analytically; and PA1 c. Calculation by the following approximate expression: EQU T(t)=Tw+(Tr-Tw).multidot.(8/.pi..sup.2).multidot.exp(-.alpha..multidot..pi. .sup.2 .multidot.t.sup.2 /R.sup.2) PA1 T(t) represents a sectional-direction average value of the in-mold resin temperature; PA1 Tw represents an average value of the metal-mold temperature (=(Twf+Twm)/2, Twf representing the fixed-side metal-mold temperature, Twm representing the movable-side metal-mold temperature); PA1 Tr represents the injected-resin temperature; PA1 .alpha.=K/(.rho..multidot.Cp); PA1 K represents the thermal conductivity of the molding material; PA1 .rho. represents the density of the molding material; PA1 Cp represents the specific heat of the molding material; PA1 t represents the point of time to be a subject of calculation; and PA1 R represents the thickness of a molded product. PA1 Tws represents the metal-mold temperature in a shot in which a good product was molded; PA1 Tr represents the resin temperature in the resin paths (2a, 3c, 3d, 3e, 3f); PA1 Trs represents the resin temperature in the resin paths (2a, 3c, 3d, 3e, 3f) in a shot in which a good product was molded; and PA1 t represents a point of time. PA1 Twms represents the movable-side metal-mold reference temperature; PA1 Trs represents the injected-resin reference temperature; PA1 R represents the thickness of a molded product; PA1 K represents the thermal conductivity of molding material; PA1 h represents the heat transfer coefficient between the molding material and the metal-mold wall surface; PA1 .rho. represents the density of the molding material; PA1 Cp represents the specific heat of the molding material; PA1 x represents the position to be a subject of calculation; PA1 .alpha.=K/(.rho..multidot.Cp); PA1 A=(1-S/h).multidot.(Twms-Twfs); PA1 B=-(S/K).multidot.(Twms-Twfs); PA1 S=1/(2/h+R/K); PA1 tan(nj.multidot.R/2)=(h/K)/nj; PA1 Dj=4.multidot.(h/K).sup.2 .multidot.{Trs-(Twfs+Twms)/2}/[nj.multidot.{nj.sup.2 +(h/K).sup.2 .multidot.R+2(h/K)}]; PA1 N represents the number of repetitions of series; and PA1 t represents the point of time to be a subject of calculation; PA1 Twf represents the measured fixed-side metal-mold temperature; PA1 Twm represents the measured movable-side metal-mold temperature; PA1 Tr represents the measured injected-resin temperature; PA1 .DELTA.Twf=Twf-Twfs; PA1 D=Twms-Twfs; PA1 .DELTA.D=(Twm-Twf)-(Twms-Twfs); PA1 .DELTA.Tr=Tr-Trs; PA1 .differential.T/.differential.Twf={Trs-Ts(t,x)-(R/2-x).multidot..xi..sub.2 .multidot.D}/(Trs-Tws); PA1 .differential.T/.differential.D=[(.xi..sub.1 +x.multidot..xi..sub.2).multidot.(Trs-Tws)-(1/2).multidot.{Ts(t,x)-Twfs -(.xi..sub.1 +x.multidot..xi..sub.2).multidot.D}]/(Trs-Tws); PA1 .differential.T/.differential.Tr={Ts(t,x)-Twfs-(.xi..sub.1 +x.multidot..xi..sub.2).multidot.D}/(Trs-Tws); PA1 .xi..sub.1 =(1+h.multidot.R/K)/(2+h.multidot.R/K); PA1 .xi..sub.2 =-(h/K)/(2+h.multidot.R/K); and PA1 Tws=(Twms+Twfs)/2
where:
Of the above-mentioned conventional methods, the method (1) requires a temperature sensor, and thus is not economical. In addition, if the in-mold resin temperature in the holding pressure and cooling stage cannot be obtained in the filing stage of injecting melted resin into the mold before the holding pressure and cooling stage, it is difficult to control the behavior of resin in holding pressure and cooling stage in which the resin in the mold is almost being solidified.
In the method (2), on the other hand, because of the complicated calculation, it takes many seconds or even several minutes to obtain an estimated value (a and b), and because of approximation errors, the accuracy is low (c).